The Experts below are selected from a list of 222 Experts worldwide ranked by ideXlab platform
Peter W. F. Smith - One of the best experts on this subject based on the ideXlab platform.
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The Manifest Association Structure of the Single-Factor Model: Insights from Partial Correlations
Psychometrika, 2007Co-Authors: Maria Salgueiro, Peter W. F. Smith, John W. McdonaldAbstract:The Association Structure between manifest variables arising from the single-factor model is investigated using partial correlations. The additional insights to the practitioner provided by partial correlations for detecting a single-factor model are discussed. The parameter space for the partial correlations is presented, as are the patterns of signs in a matrix containing the partial correlations that are not compatible with a single-factor model.
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MEANINGFUL REGRESSION AND Association MODELS FOR CLUSTERED ORDINAL DATA
Sociological Methodology, 2006Co-Authors: Jukka Jokinen, John W. Mcdonald, Peter W. F. SmithAbstract:Many proposed methods for analyzing clustered ordinal data focus on the regression model and consider the Association Structure within a cluster as a nuisance. However, the Association Structure is often of equal interest—for example, temporal Association in longitudinal studies and Association between responses to similar questions in a survey. We discuss the use, appropriateness, and interpretability of various latent variable and Markov models for the Association Structure and propose a new Structure that exploits the ordinality of the response. The models are illustrated with a study concerning opinions regarding government spending and an analysis of stability and change in teenage marijuana use over time, where we reveal different behavioral patterns for boys and girls through a comprehensive investigation of individual response profiles.
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Association-Marginal Modeling of Multivariate Categorical Responses: A Maximum Likelihood Approach
Journal of the American Statistical Association, 1999Co-Authors: Joseph B. Lang, John W. Mcdonald, Peter W. F. SmithAbstract:Abstract Generalized log-linear models can be used to describe the Association Structure and/or the marginal distributions of multivariate categorical responses. We simultaneously model the Association Structure and marginal distributions using Association-marginal (AM) models, which are specially formulated generalized log-linear models that combine two models: an Association (A) model, which describes the Association among all the responses; and a marginal (M) model, which describes the marginal distributions of the responses. Because the model's composite link function is not required to be invertible, a large class of models can be entertained and model specification is typically straightforward. We propose a “mixed freedom/constraint” parameterization that exploits the special Structure of an AM model. Using this parameterization, maximum likelihood fitting is straightforward and typically feasible for large, sparse tables. When a parsimonious Association model is used, the size of the fitting proble...
Dimitris Rizopoulos - One of the best experts on this subject based on the ideXlab platform.
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extension of the Association Structure in joint models to include weighted cumulative effects
Statistics in Medicine, 2017Co-Authors: Katya Mauff, Ewout W Steyerberg, Giel Nijpels, Amber A Van Der Heijden, Dimitris RizopoulosAbstract:Motivated by a study measuring diabetes-related risk factors and complications, we postulate an extension to the standard formulation of joint models for longitudinal and survival outcomes, wherein the longitudinal outcome has a cumulative effect on the hazard of the event, weighted by recency. We focus on the relationship between the biomarker HbA1c and the development of sight threatening retinopathy, since the impact of the HbA1c marker on the risk of sight threatening retinopathy is expected to be cumulative, with the evolution of the HbA1c marker over time contributing to progressively greater damage to the vascular Structure of the retina. Opting for a parametric approach, we propose the use of the normal and skewed normal probability density functions as weight functions, estimating the relevant parameters directly from the data. The use of the recency-weighted cumulative effect specification allows us to incorporate differences in the development of the longitudinal profile over time in the calculation of hazard ratios between subjects. The proposed functions provide us with parameters with clinically relevant interpretations while retaining a degree of flexibility. In addition, they also allow answering of important clinical questions regarding the relative importance of various segments of the biomarkers history in the estimation of the risk of the event. Copyright © 2017 John Wiley & Sons, Ltd.
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Combining Dynamic Predictions From Joint Models for Longitudinal and Time-to-Event Data Using Bayesian Model Averaging
Journal of the American Statistical Association, 2014Co-Authors: Dimitris Rizopoulos, Laura A. Hatfield, Bradley P. Carlin, Johanna J.m. TakkenbergAbstract:The joint modeling of longitudinal and time-to-event data is an active area of statistics research that has received a lot of attention in recent years. More recently, a new and attractive application of this type of model has been to obtain individualized predictions of survival probabilities and/or of future longitudinal responses. The advantageous feature of these predictions is that they are dynamically updated as extra longitudinal responses are collected for the subjects of interest, providing real time risk assessment using all recorded information. The aim of this article is two-fold. First, to highlight the importance of modeling the Association Structure between the longitudinal and event time responses that can greatly influence the derived predictions, and second, to illustrate how we can improve the accuracy of the derived predictions by suitably combining joint models with different Association Structures. The second goal is achieved using Bayesian model averaging, which, in this setting, ha...
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Joint modelling of longitudinal and survival data.
2008Co-Authors: Dimitris RizopoulosAbstract:A common objective in longitudinal studies is the investigation of the Association Structure between a longitudinal response process and the time to an event of interest. An attractive paradigm for the joint modelling of longitudinal and survival processes is the shared parameter framework where a set of random effects is assumed to induce their interdependence. In this work, we propose an alternative parameterization for shared parameter models and investigate the effect of misspecifying the random effects distribution in the parameter estimates and their standard errors.
Toshiro Masuda - One of the best experts on this subject based on the ideXlab platform.
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Influence of alkaline concentration on molecular Association Structure and viscoelastic properties of curdlan aqueous systems
Biopolymers, 1997Co-Authors: Toshio Tada, Takayoshi Matsumoto, Toshiro MasudaAbstract:Curdlan is an extracellular polysaccharide produced from soil microorganism Alcaligens faecalis var. 10C3K, and the linear Structure consists of β-1,3-glycoside linkages. Curdlan is not soluble in water but it is soluble in alkaline aqueous solution, and we can obtain the gel when curdlan alkaline solution is heated above 60°C or neutralized by acids. In the present study, the gelation mechanism and dispersing Structure of curdlan in the alkaline solutions are studied in terms of correlation between the molecular Association Structure and viscoelastic properties, using static light scattering and rheological measurements. The degree of Association for the curdlan molecules in dilute solution increases with decreasing alkaline concentration. The viscoelastic properties also depend strongly on the alkaline concentration. The concentrated curdlan solution shows almost Newtonian flow at high alkaline concentrations and shows a gel-like behavior at low alkaline concentrations. It was elucidated that the molecular Association in the dilute solution reflects on the viscoelastic properties of the concentrated solution and that the gelation mechanism is related to the Association Structure of curdlan molecules. In the case of lower NaOH concentration systems, the molecular Association is likely to consist of a hydrophobic core and hydrophilic surface. The gelation mechanism above 60°C is considered to include the dissociation process of the molecular Association and reformation of the network Structure. © 1997 John Wiley & Sons, Inc. Biopoly 42: 479–487, 1997
John W. Mcdonald - One of the best experts on this subject based on the ideXlab platform.
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The Manifest Association Structure of the Single-Factor Model: Insights from Partial Correlations
Psychometrika, 2007Co-Authors: Maria Salgueiro, Peter W. F. Smith, John W. McdonaldAbstract:The Association Structure between manifest variables arising from the single-factor model is investigated using partial correlations. The additional insights to the practitioner provided by partial correlations for detecting a single-factor model are discussed. The parameter space for the partial correlations is presented, as are the patterns of signs in a matrix containing the partial correlations that are not compatible with a single-factor model.
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MEANINGFUL REGRESSION AND Association MODELS FOR CLUSTERED ORDINAL DATA
Sociological Methodology, 2006Co-Authors: Jukka Jokinen, John W. Mcdonald, Peter W. F. SmithAbstract:Many proposed methods for analyzing clustered ordinal data focus on the regression model and consider the Association Structure within a cluster as a nuisance. However, the Association Structure is often of equal interest—for example, temporal Association in longitudinal studies and Association between responses to similar questions in a survey. We discuss the use, appropriateness, and interpretability of various latent variable and Markov models for the Association Structure and propose a new Structure that exploits the ordinality of the response. The models are illustrated with a study concerning opinions regarding government spending and an analysis of stability and change in teenage marijuana use over time, where we reveal different behavioral patterns for boys and girls through a comprehensive investigation of individual response profiles.
-
Association-Marginal Modeling of Multivariate Categorical Responses: A Maximum Likelihood Approach
Journal of the American Statistical Association, 1999Co-Authors: Joseph B. Lang, John W. Mcdonald, Peter W. F. SmithAbstract:Abstract Generalized log-linear models can be used to describe the Association Structure and/or the marginal distributions of multivariate categorical responses. We simultaneously model the Association Structure and marginal distributions using Association-marginal (AM) models, which are specially formulated generalized log-linear models that combine two models: an Association (A) model, which describes the Association among all the responses; and a marginal (M) model, which describes the marginal distributions of the responses. Because the model's composite link function is not required to be invertible, a large class of models can be entertained and model specification is typically straightforward. We propose a “mixed freedom/constraint” parameterization that exploits the special Structure of an AM model. Using this parameterization, maximum likelihood fitting is straightforward and typically feasible for large, sparse tables. When a parsimonious Association model is used, the size of the fitting proble...
Toshio Tada - One of the best experts on this subject based on the ideXlab platform.
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Influence of alkaline concentration on molecular Association Structure and viscoelastic properties of curdlan aqueous systems
Biopolymers, 1997Co-Authors: Toshio Tada, Takayoshi Matsumoto, Toshiro MasudaAbstract:Curdlan is an extracellular polysaccharide produced from soil microorganism Alcaligens faecalis var. 10C3K, and the linear Structure consists of β-1,3-glycoside linkages. Curdlan is not soluble in water but it is soluble in alkaline aqueous solution, and we can obtain the gel when curdlan alkaline solution is heated above 60°C or neutralized by acids. In the present study, the gelation mechanism and dispersing Structure of curdlan in the alkaline solutions are studied in terms of correlation between the molecular Association Structure and viscoelastic properties, using static light scattering and rheological measurements. The degree of Association for the curdlan molecules in dilute solution increases with decreasing alkaline concentration. The viscoelastic properties also depend strongly on the alkaline concentration. The concentrated curdlan solution shows almost Newtonian flow at high alkaline concentrations and shows a gel-like behavior at low alkaline concentrations. It was elucidated that the molecular Association in the dilute solution reflects on the viscoelastic properties of the concentrated solution and that the gelation mechanism is related to the Association Structure of curdlan molecules. In the case of lower NaOH concentration systems, the molecular Association is likely to consist of a hydrophobic core and hydrophilic surface. The gelation mechanism above 60°C is considered to include the dissociation process of the molecular Association and reformation of the network Structure. © 1997 John Wiley & Sons, Inc. Biopoly 42: 479–487, 1997
